Short answer: annual RV generator service runs $200-$450 for routine maintenance (oil, filter, plugs). Repair costs vary: $350-$700 for carb cleaning, $400-$900 for voltage regulator, $300-$650 for brush replacement on older units, and $5,500-$11,000 for full replacement. Cummins Onan generators last 4,000-8,000 hours with proper service. Without service, half that. Here is the maintenance schedule and what each service actually involves.
What runs in your RV (likely)
About 80 percent of RVs roll out of the factory with Cummins Onan generators. The QG series (3,200-7,000W) and HDKCA (5,500-12,500W) are by far the most common. Generac is gaining ground, especially in newer rigs. Older units may have Briggs & Stratton or others.
Knowing your model tells us how to service it and what parts to source.
The service intervals that matter
| Service | Interval | Cost |
|---|---|---|
| Oil change | Every 150 hours or annually | $100-$200 |
| Air filter | Every 150 hours or annually | $30-$80 |
| Fuel filter | Every 300 hours or 2 years | $40-$120 |
| Spark plug | Every 500 hours or 3 years | $25-$60 |
| Full annual service (combined) | Annually | $200-$450 |
| Brush inspection/replacement (carbureted units) | 500-1000 hours | $300-$650 |
| Carb cleaning/rebuild | As needed (won't start) | $350-$700 |
| Voltage regulator | As needed (output issues) | $400-$900 |
The 5 most common generator problems
1. Won't start
Symptoms: cranks but won't fire, or doesn't crank at all. Causes: stale fuel (most common after winter storage), clogged carb, bad spark plug, fuel pump issue, battery problem.
Diagnostic: $150-$300. Fix typically $200-$700 depending on cause.
2. Runs but no power output
Generator runs but A/C, outlets, and appliances don't get power. Causes: voltage regulator failure, brush wear (older units), capacitor failure, transfer switch problem.
Diagnostic + repair: $400-$1,200 typical.
3. Low voltage output
Output is there but it's reading 95-105V instead of 120V. Causes: voltage regulator drift, brush wear, frequency issue (low RPM).
Fix: $400-$900 most cases.
4. Runs rough or shuts down under load
Starts fine, runs at no load fine, A/C kicks on and generator dies. Causes: insufficient fuel flow, dirty carb, governor adjustment, valve adjustment needed.
Fix: $250-$700.
5. Excessive smoke
Blue smoke: burning oil (worn rings or valves). $1,500-$4,000 to repair, often economical to replace at this point.
Black smoke: rich mixture (carb or fuel pressure issue). $300-$700.
White smoke at startup: condensation, usually fine. White smoke continuing: head gasket or worse.
When to replace vs repair
Rule of thumb: when repair cost exceeds 40 percent of replacement cost, replace. A new Onan QG 7000 retails around $5,500-$7,000 installed. If a repair quote is $2,500+, hard look at replacement.
Replacement generator cost (parts + labor):
- QG 4000 (small motorhome): $3,500-$5,500
- QG 5500: $4,500-$7,000
- QG 7000: $5,500-$8,500
- HDKCA 10K-12K (Class A diesel): $9,000-$13,000
Why generators die early
Three things that kill RV generators faster than necessary:
- Sitting unused. Stale fuel gums up the carb. Run the generator under load at least 30 minutes once a month.
- No oil changes. Generator oil works harder than vehicle oil. Change at the schedule.
- Running with low fuel. Onan generators auto-shut at 1/4 tank to protect the engine. Some owners disable this. Don't.
DIY vs shop
DIY: oil change, air filter, spark plug, basic visual inspection. All reasonable for someone with mechanical comfort.
Shop: carb work, electrical diagnostics, voltage regulator, brush replacement, anything requiring multimeter and load testing. Bring it to us.
Get a real quote
Describe your generator model and the problem. Get a service quote or call 719-722-2537.
Related: RV repair costs overview, cost calculator, Mile High RV Works overview.
Frequently asked questions
How often should I service my Onan or Cummins RV generator?
Manufacturer recommendation is every 150 hours of run time or annually, whichever comes first. Heavy boondocking users hit 150 hours fast (a typical week of dry camping with AC adds 50 to 70 hours). Light users still need the annual service because oil and filters degrade by calendar even when not run.
Why won't my Onan generator start?
Most common: stale gasoline (carburetor gunked from sitting), a dead start battery (separate from the house bank), a clogged fuel filter, or a bad spark plug. About 80% of 'won't start' calls we get are one of those four. The fix is usually a same-day job once we get hands on it.
How long does standard generator service take?
Three to five hours of bench time for a standard service (oil, oil filter, fuel filter, air filter, spark plug, brush inspection, load test, fault scan). Often a same-day drop-off if you schedule ahead. Major repairs (carburetor rebuild, starter, voltage regulator) take longer and depend on parts availability.
Should I repair or replace an old RV generator?
Replace if the unit is over 15 years old, has been allowed to sit dry for years, or if the repair cost approaches half the cost of a new unit. Repair if the unit ran reliably until recently and the failure is in a serviceable component (carburetor, starter, regulator). We will give you the honest call after inspection, not push a replacement that does not make sense.
Will my generator work at altitude in Colorado?
Yes, but with reduced output. Naturally aspirated generators lose roughly 3% of rated output per 1,000 feet above sea level. A 4kW unit in Denver puts out closer to 3.4 kW. Above 7,000 feet (Frisco, Leadville), derating is significant and you may need to limit which appliances run simultaneously. We can adjust carb settings to optimize for altitude during service.
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